Graphene is essentially an individual layer of graphite. Graphene can also be thought of as a carbon nanotube unrolled as shown in FIG. 9.
Graphene sheets offer many extraordinary properties and are being investigated for use in nanoelectronics, nanocomposites, batteries, supercapacitors, hydrogen storage and bioapplications. The main limitation for the use of graphene sheets is the current inability to mass produce them. Like carbon nanotubes and many other nanomaterials, a key challenge in the synthesis and processing of bulk-quantity graphene sheets is aggregation. Graphene sheets with high specific surface area, unless well separated from each other, tend to form irreversible agglomerates or even restack to form graphite due to van der Waals interactions. This problem has been encountered in all previous efforts aimed at large-scale production of graphene through chemical conversion or thermal expansion/reduction.
The prevention of aggregation is of particular importance for graphene sheets because most of their unique properties are only associated with individual sheets. Aggregation can be reduced by the attachment of other molecules or polymers onto the sheets. However, the presence of foreign stabilisers is undesirable for most applications. New strategies to produce relatively clean graphene sheets in bulk quantity while keeping them individually separated are required.
Graphite, consisting of a stack of flat graphene sheets, is inexpensive and available in large quantities from both natural and synthetic sources. This ordinary carbon material is the most readily available and least expensive source for the production of graphene sheets. Mechanical cleavage of graphite originally led to the discovery of graphene sheets and is currently used in most experimental studies of graphene. However, the low productivity of this method makes it unsuitable for large-scale use. Chemical conversion from graphite appears to be a much more efficient approach to bulk production of graphene sheets at low cost. The solution-based route involves chemical oxidation of graphite to hydrophilic graphite oxide, which can be readily exfoliated as individual graphene oxide sheets by ultrasonication in water. Graphene oxide, which is electrically insulating, can be converted back to conducting graphene by chemical reduction, e.g. using hydrazine. Unfortunately, previous work has shown that unless stabilised by selected polymers, chemically converted graphene (CCG) sheets obtained through this method precipitate as irreversible agglomerates due to their hydrophobic nature. The resulting graphene agglomerates appear to be insoluble in water and organic solvents, making further processing difficult.
As shown with carbon nanotubes, the dispersion of nanomaterials in solution is crucial to advancing many technological applications. Owing to their hydrophobic nature, the direct dispersion of graphite or graphene sheets in water has been generally considered unattainable.